Want to know more about rocks? These are a few of the books that I use.
I grew up with Zim's Golden Nature Guides, and they played a large role in my science adventures. This one was my first book on minerals, and along with the Golden Nature Guide to Fossils, was some of my favorite reading. It is a great book for young readers, clearly written and packed with good information. It is small enough to pack easily for field trips, and cheap enough that you can afford to buy a new copy when it eventually wears out from use.
This is an excellent reference if you are really into minerals and rocks. Much of the writing may be too technical for young readers, but it has a good glossary, so an excited and determined 5th grader would find it quite useful. It has excellent, color photographs. For each rock and mineral, it lists physical properties, how it forms, the location of major deposits worldwide, and how the mineral is used.
Unfortunately, this book is out of print, but you can still find used copies, and many libraries have a copy. The authors take a very different approach from most rock and mineral books. They start with rocks, and for each type, examine how they form and the minerals that they usually contain. This is the book that I grab when I am going on a geology field trip.
I like all of Janice's books. She writes well, and gets the science right without watering it down.
I recently discovered the "Discover Nature" series, and really like them. This one includes minerals, rocks, erosion, volcanoes, earthquakes, and much more. Each chapter has activities, suggested reading, and suggested viewing lists.